McDonald’s Corp. said Monday that continued steep declines in same-store sales in Japan and China in January outweighed a slight improvement in the U.S., extending the slump that has roiled the fast-food company.

McDonald’s said overall sales at restaurants open at least 13 months fell 1.8% in January—worse than the 1.2% decline analysts expected, according to Consensus Metrix. Same-store sales in the U.S. rose 0.4% as “aggressive competitive activity” overall offset strong breakfast sales. Following a similar uptick in...

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